This invention relates to a process for alleviation of sulfur deposition in sour gas wells and more particularly to a process for treating sour gas wells by the introduction of an alkyl naphthalene base sulfur solvent into the well.
In many cases the production of natural gas is complicated by the presence of hydrogen sulfide in the subterranean formation in which the gas is found. Natural gas containing appreciable amounts of hydrogen sulfide is commonly referred to as "sour gas." In addition to the problems caused by the toxicity of hydrogen sulfide, serious problems are presented by sulfur deposition. Under the conditions prevailing in the subterranean reservoir, the sulfur may be partially dissolved in the compressed gas or finely divided in a liquid phase and may partially or wholly chemisorbed as hydrogen polysulfide (sulfane). In any event, the decrease in temperature and pressure attendant to the upward flow of gas as it is produced to the surface, as well as catalytic effects, result in the separation of elemental sulfur from the gas. A solid precipitate of sulfur may form in certain critical places such as on the interior wall of the production string, thus restricting or actually plugging the flow passageway.
Various processes have been proposed for the prevention or removal of such sulfur precipitates. These include intermittently applied measures such as mechanical removal of sulfur deposits or the batchwise application of liquids having high sulfur absorption power. Such measures of necessity involve interrupting the operation of the well. The more widely applied procedures involve the continuous processes. In these procedures, sulfur deposition is prevented by absorbing elemental sulfur by chemical or physical dissolution in a medium which is produced with the gas stream and is then separated from the gas stream by any suitable technique. There are many substances which in the liquid phase are useful in dissolving sulfur. However, in order to be useful in sour gas wells, such substances should meet a number of criteria. They should be capable of rapid binding of sulfur and should be nonflammable and nontoxic. The solvents should not damage the formation and should exhibit chemical and thermal stability under the process conditions. In addition, they should be compatible with the downstream surface equipment and capable of regeneration without excessive losses. Suitable solvents also should be readily separable from the produced gas and resistant to emulsification with entrained water and normally should have a density differential relative to water of at least 0.05.
German Pat. Nos. 1,157,569, 1,164,345, and 1,296,586, and U.S. Pat. Nos. 3,223,156 and 3,909,422 disclose, for example, the use of alkali, ammonia or amine solutions which in the presence of hydrogen sulfide form sulfides and bind elemental sulfur as polysulfide. Surface-active agents also may be added in order to improve the sulfur binding properties. The alkali, ammonia, or amine sulfides function to dissolve sulfur; but if carbon dioxide is present in the sour gas, as is often the case, the corresponding carbonates are also formed. Thus, the application of these solvents is limited to sour gas wells having a relatively favorable H.sub.2 S:CO.sub.2 ratio of about 0.7 to 1.5. Moreover, these processes are not applicable where formation waters produced by the well contain alkaline-earth metal chlorides such as calcium and magnesium chlorides. In this case where the alkaline aqueous medium also contains carbon dioxide, calcium carbonate and magnesium hydroxide deposits may also result in plugging of the production equipment.
Solvents which have heretofore been proposed for use in preventing sulfur deposition include liquid hydrogen sulfide (U.S. Pat. No. 3,393,733), carbon disulfide (Canadian Pat. No. 771,129), organic sulfides and disulfides (U.S. Pat. No. 3,531,160) and disulfides (German published patent application No. 2,422,098). These processes involve disadvantages due to the dangers in handling the chemicals and difficulties in gas-processing plants because of the formation of nonregenerable products in purification processes using alkanolamines or because of the high volatility of the substances employed. The use of tetraline as a sulfur solvent is disclosed in German Pat. No. 1,225,977. However, this process presents difficulties because of the attendant high vapor pressure and the reaction under borehole conditions with elemental sulfur to produce naphthalene.
German Pat. No. 1,173,856 discloses the use of high-boiling mineral oil hydrocarbon mixtures of spindle oil type. Relatively large quantities of such solvents are required because of the relatively low solubility of sulfur therein. In regenerating the solvent, the dissolved sulfur is extracted by scrubbing with alkali hydrogen sulfide solutions or alkanolamine solutions (German published patent application No. 2,707,057). In this procedure, surface-active substances are used as emulsion breakers and to facilitate the transfer of sulfur from the organic phase into the aqueous phase.
Hydrocarbon solvents such as benzene, toluene, xylene, kerosene or diesel oil are not generally useful in the alleviation of sulfur deposition because of the high volatility of these substances. Other possible solvents such as hydrocarbon halides as well as various esters are hydrolyzed under borehole conditions and form highly corrosive decomposition products. Certain polycyclic aromatics such as naphthalene, .alpha.-, or .beta.-naphthol, and anthracene have relatively high melting points, exhibit a tendency to form deposits in low-temperature portions of the production system, or have unacceptably high vapor pressures. Hydrocarbon mixtures such as coal tar oils and various high-boiling mineral oil fractions tend to undergo decomposition reactions in the presence of elemental sulfur which result, among other things, in the formation of insoluble asphalt-like products and in the formation of carbon sulfide and carbonyl sulfide. The carbon sulfide and carbonyl sulfide present difficulties in the downstream treatment and processing system and the higher-molecular weight decomposition products cause difficulty in the regeneration of solvents and, in addition, may act as plugging agents.